1. Field Of the Invention
This invention relates to a high molecular weight copolymer of ethylene and a C.sub.3-20 .alpha.-olefin. More especially this invention relates to such a high molecular weight copolymer which can be molded at room temperature and, after pressure-free tempering at 120-250.degree. C. followed by cooling, provides a molded article having ultimate tensile strength of more than 200 kg per square cm measured at 120.degree. C. and an impact strength of more than 200 kg cm/cm.sup.2 measured at 23.degree. C. This invention further relates to a process for the manufacture of such a high molecular weight ethylene-C.sub.3-20 .alpha.-olefin copolymer. Furthermore, this invention relates to a catalyst useful in the preparation of such a high molecular weight copolymer.
2. Discussion Of The Prior Art
It is known that ethylene or other olefins, alone or as a mixture, can be polymerized at low pressure in the presence of catalysts. The latter consist of mixtures of the metals of Group 4A of the periodic system and organoaluminum compounds (Ziegler Process). The procedure allows one to manufacture, among other products, high molecular polymerizates and copolymerizates of ethylene with molecular weights of 500,000 and greater, e.g., 5,000,000 to 8,000,000. The molecular weights reported herein are determined by measuring the viscosity of the polymer solutions.
The high molecular weight polymers are usually in powder form and are converted into molded articles of various types by appropriate processing. These molded articles are of special value when the following properties are required: abrasion resistance, impact strength, resistance to solvents and corrosive chemicals.
Some examples of these types of applications are heavy duty coverings for containers, pipe lines and pieces of apparatus as well as for fast moving machine parts of various types (alone or in combination with other materials e.g., steel), which are exposed to abrasion. They are also useful in the manufacture of artificial limbs and implants.
The manufacture of molded articles from high molecular polyethylene or copolymerizates of ethylene takes place mainly by means of heat compression molding or by extruding the powder to sheets, blocks or sections, from which, if necessary, thinner sheets and smaller molded articles can be made by machining.
This type of processing is very involved and due to the lengthy period which the material is in the mold, under high pressure and at high temperature, only a low rate of manufacture is possible. Moreover, during the machining there is a considerable loss of material.
A simpler method for forming high molecular polyethylene powder has already been attempted as in U.S. Pat. No. 3 847 888. In this instance the powder from the synthesis was ground down to a grain size less than 100 .mu.. Powders of this type can be molded at room temperature and by subsequent heat treatment converted into completely homogeneous sintered molded articles. Thus, the residence time in the molding press can be reduced and the heating during the molding procedure eliminated. By finely adjusting the mold to the final dimensions of the molded article, the loss of material from any necessary remachining can be minimized.
However, this known procedure results in high costs due to the involved grinding process. Moreover, completely homogeneously molded articles are not always obtained. Particularly with larger molded articles, there is the risk of bubble formation, which impairs the physical properties of the article. Finally, such fine powders are difficult to handle due to the associated dust problems and can even be dangerous under certain circumstances.
It is, therefore, an object of the invention, to provide a process for the manufacture of molded articles from a more easy workable high molecular polymerizate powder which process requires less mechanical and thermal processing, is performed over a shorter period of time and produces molded articles possessing high ultimate tensile and impact strengths.